1. Field of the Invention
This invention is in the field of reducing the sulfur content of coal in a continuous, integrated process involving grinding the coal, activating it thermally, chemically, or otherwise, to make the pyrite constituent more magnetically susceptible and then separating the activated pyrite magnetically from the remainder of the coal.
2. Description of the Prior Art
The desulfurization of coal has been a matter of concern for a long period. Research of the Bergbau-Forschung GmbH of North Rhine Westfalia has determined that about 60% of the total sulfur of the coal recovered in the Ruhr coal district is bound to iron and of this amount approximately 75% is present as free pyrite or pyrite loosely combined with coal and mine waste. Accordingly, on an average up to about 40% of the total sulfur of such coal is regarded as separable by proper procedures.
Research steps have further revealed that almost 80% of the separable pyrite is present in a grain size which is coarser than 0.06 mm. Because the density of the pyrite is at an average of about 5, that of the mine waste is about 2.5, and that of the coal about 1.3 kg/dm.sup.3, various separation procedures involving a comminution of the coal and gravity sorting methods have been used to effect such separation. For example, with a grain size range of 0 to 10 mm, it was possible to separate on the fine grain settling machines barely 40% of the pyrite sulfur in the tailings. The sulfur content of the raw coal could thereby be reduced by about 10%.
With finer grain fractions, the most favorable results were obtained with flotation, whereby a reduction of pyrite sulfur up to 65% was attained, corresponding to a lowering of the total sulfur content by about 50%. Flotation processes, however, are usable only with grain sizes less than 1 mm.
For grain sizes in the range of 0.06 mm to 3 mm, suitable results can be obtained through the use of oscillating tables where in the most favorable case, there can be obtained a removal of pyrite of about 60% and a total sulfur removal corresponding to approximately 40%.
In the case of very fine grain materials, separation by means of a water cyclone has also been successful, whereby in some cases a total sulfur removal of approximately 50% could be obtained.
In any of these cases, however, there was a disadvantage that there was a relatively high loss in convertible coal in the removal of the sulfur-rich concentrate.
Any wet separation method is not desirable where the coal is to be used, for example, in the operation of power plants where the coal is blown as a dust directly into the combustion chamber in dry condition. Because of this, it was also attempted to carry out the treatment of the coal in a dry manner, for example, through sorting in a magnetic field. The basis for this separation is the fact that pure pyrite (Fe S.sub.2) which is either diamagnetic or weakly paramagnetic may be converted into ferromagnetic or at least more strongly paramagnetic through activation to increase its magnetic susceptibility by a factor of 10 to 20. This can occur, for example, by means of reduction of the pyrite to pyrrhotin (Fe.sub.7 S.sub.8) or also through the oxidation of the pyrite to Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4.
A known method of this type is called the thermal activation of pyrite and uses treatment temperatures in the range of 300.degree. to 500.degree. C., and preferably at 400.degree. C. On heating a pyrite sample in an electric resistance furnace, the magnetic susceptibility remains approximately constant up to about 300.degree. C. Beyond this, the susceptibility rises greatly so that at a temperature of about 380.degree. C., the susceptibility is increased by a factor of about 20. The sulfur-iron combination so treated is well adapted to sorting according to differences in magnetic susceptibility.
A disadvantage of this process, however, is the difficulty which results because the coal may contain substantial amounts of readily volatile constituents which may be lost or may present a disposal problem when the coal is heated to temperatures of about 400.degree. C. Furthermore, the cost of operation and capital expenses are substantial for this type of process.